Electrochemical fuel cells employing membrane electrode assemblies are known and have been produced and sold for many years. Such cells are known as solid polymer type fuel cells, and comprise, in the heart of the system, two porous electrodes separated by an electrolyte material. The porous electrodes, conveniently made from carbon fiber paper ("CFP") and a catalyst such as platinum, and the electrolyte material in the form of a membrane together form an assembly called a membrane electrode assembly ("MEA"). The MEA is located between two electrically conductive or, conveniently, graphite flow field plates. The graphite flow field plates supply fuel and oxidant in the form of hydrogen and air or oxygen, respectively, to the MEA and also act to provide current generated by the fuel cell to an external electrical circuit where it may be stored or otherwise used. The fuel and oxidant are supplied to the MEA by grooves in the surface of the graphite flow field plates adjacent the carbon fiber paper. The grooves communicate via manifolds carrying gases to each of the individual MEAs.
The assembly includes a catalytic material, conveniently platinum as aforesaid, on the surface of the CFP which renders the CFP an electrode. Each electrode portion of the CFP contacts the membrane. The CFP of the electrode is made hydrophobic. Ridges between the grooves in the graphite flow field plates contact the back of the electrode portion of the CFP. The MEA consumes the fuel and oxidant through an electrochemical process and produces an electrical current which can be drawn from the electrodes to an external circuit.
To ensure that the fuel and oxidant gases supplied to the MEA do not mix, sealing to prevent such mixing is imperative. In the event that hydrogen and oxygen combine within the fuel cell in combination with the catalyst, a combustible mixture can form and inflame. In the event that the fuel and oxidant leak from the interior to the exterior of the fuel cell, it can reduce the efficiency of the fuel cell and create a fire or explosive hazard.
In conventional fuel cells, an MEA was used between the two electrically conductive, preferably graphite, plates which included a membrane bonded between the CFP layers. The membrane extended substantially beyond the edge or periphery of the CFP layers and was not supported by or bonded to them, and the CFP layers covered only the inner or active portion of the membrane. The outer periphery of the membrane was free of the CFP.
This conventional membrane electrode assembly was disadvantageous in several respects. First, the membrane was installed between two adjacent electrically conductive plates and acted as a gasket sealing the gases in the electrode region from the exterior, isolating the gases in their respective manifolds and electrically insulating the electrical conducting flow field plates between which it was installed.
Such a configuration for the membrane, however, did not function well as a gasket. The membranes were subject to shrinking and swelling depending on the water content of the membrane. Since they were free to shrink and swell, the potential for tearing or for fatigue cracks to form was high. Although various techniques were utilized in an attempt to minimize the leaks across the membrane between the flow field plates, the techniques were expensive and substantially ineffective over an extended time period.
These applications of the membrane electrolyte, where the unsupported outer edges of the membrane serve as the insulator and gasket between the opposing flow field plates, place strength and resilience demands upon the membrane which then limit the minimum thickness of electrolyte which can realistically be used in a fuel cell. At a thickness below about 0.005 inches, the typical membrane material is too fragile to withstand the compressive forces required for sealing. Such thin membranes are subject to tearing when the cell is assembled or disassembled and when the membrane electrolyte is cycled between the hydrated operating state and the dehydrated non-operating state.
Up to a point, it is desirable to reduce the thickness of the membrane electrolyte as the electrolyte represents a substantial component of the internal electrical resistance of the fuel cell. A fuel cell with a thinner electrolyte will have a lower internal resistance and thus a higher voltage will be available at the fuel cell terminals for a given current demand. This translates directly into a greater power and fuel efficiency being derived from a fuel cell with a thinner electrolyte. This improvement is balanced only by the requirement that the membrane be sufficiently thick to sustain the operating pressure differential between the fuel and oxidant gases and to minimize the diffusional mixing of these two gas streams through the membrane.
With the unsupported membrane of the conventional membrane electrode assembly, it was also necessary to machine a recess in each flow field plate contiguous with the periphery of the CFP so that the MEA could be appropriately positioned between the flow field plates with a uniform distance being maintained about the periphery so that the membrane could be tightened appropriately between the plates and function with a good sealing action. Such machining was time consuming and expensive and, in fact, did not assist substantially in enhancing the sealing action.
Yet a further disadvantage with the conventional membrane electrode assembly was that the membrane itself was difficult to position and maintain in position while the stack assembly was being assembled. This was so since the membrane is quite thin and is inherently very flexible. It was also being subject to expanding and contracting due to the humidity changes in the gases to which the membrane was subjected.
Yet a further disadvantage in the prior membrane electrode assembly was the problem of positioning seals about the water and gas passages which extended through the flow field plates and the membrane. This was accomplished by machining grooves in the graphite flow field plates on either side of the membrane assembly and manually positioning rubber seals in the grooves. This was time consuming and, when assembling the cells, the seals could be dislodged from the grooves if of an O-ring configuration. If the seals took a rectangular configuration, they could be rolled in their grooves. In either case, the sealing action was adversely affected.